1. Field of the Invention
The present invention relates to a die cushion mechanism, and also to an apparatus and a method for controlling the die cushion mechanism.
2. Description of the Related Art
In a press machine for performing press working, such as bending, drawing, stamping, etc., it is known that a die cushion mechanism is provided as an auxiliary apparatus for applying, during a pressing operation, a required force (or pressure) to a movable support member (generally referred to as a slide) supporting a first die used for the press working, from the side of a second support member (generally referred to as a bolster) supporting a second die. The die cushion mechanism is usually constructed such that a movable element (generally referred to as a cushion pad) retained under a predetermined pressure is arranged to collide directly or indirectly with the slide (or the first die) moving in a die-closing direction and, after the collision, the cushion pad moves together with the slide while exerting a force (or pressure) to the slide, through the step of die-closing (or press forming) to the step of die-opening. During this procedure, it is possible, for example, to prevent a material to be pressed (or a workpiece) from being wrinkled, by sandwiching the peripheral region of the workpiece surrounding a pressed area between the cushion pad and the slide.
In order to improve the accuracy of the press working using the die cushion mechanism, it is required that the cushion pad stably applies a commanded force (or pressure) to the slide during a period when the cushion pad is moving together with the slide. However, as a conventional die cushion mechanism uses a hydraulic or pneumatic device as the drive source, it has generally been difficult to control the force (or pressure) applied to the slide so as to correspond to a command value in a variable mode, in response to a sudden pressure variation due to external causes, such as the collision with the slide, etc. Therefore, a die cushion mechanism including a servo-motor as a drive source has been recently developed, so as to achieve force control with an excellent performance in response.
For example, Japanese Unexamined Patent Publication (Kokai) No. 10-202327 (JP-A-10-202327) discloses a die cushion mechanism in which a cushion pad arranged beneath a slide of a press machine is vertically moved up and down by a servo-motor so as to correspond to the vertical motion of the slide. During a period when the slide is moved downward (i.e., during the pressing operation), and before the slide applies a collision force to the cushion pad, the servo-motor acts in accordance with a position control based on a position command for the cushion pad, so as to locate the cushion pad at a predetermined waiting position. Also, after the slide applies a collision force to the cushion pad, the servo-motor acts in accordance with a force control based on a force command previously determined to correspond to the position of the cushion pad, so as to move the cushion pad together with the slide and simultaneously adjust the force (or pressure) applied to the slide from the cushion pad. In this connection, the detection of collision and pressure is accomplished by detecting a load applied to the output shaft of the servo-motor through the cushion pad.
As described above, in the conventional die cushion mechanism using a servo-motor drive, the force (or pressure) applied to the slide from the cushion pad is suitably adjusted by changing the control scheme of the servo-motor from the position control to the force control at an instant when the slide exerts a collision force on the cushion pad. However, it is difficult, with only such a simple change of the control scheme, to properly control the force (or pressure) of the cushion pad quickly in response to a significant pressure fluctuation due to the impact of collision.
For example, in the case of the force control, the speed or torque of the servo-motor is ultimately controlled, so that a speed command to the servo-motor is always required, due to a speed feedback from the servo-motor that arises during the execution of the force control (i.e., during the movement of the cushion pad). Therefore, a compensator such as an integrator needs to be used to hold the speed command. However, the response of an integrator is generally not fast enough, and there may be a case that it is difficult to hold a proper speed command following a rapid fluctuation of the force at the time of collision. In this case, the overshoot of the force immediately after collision may become excessively large. Also, the position control is changed to the force control usually with a force command value of the force control being held at a certain value, so that, similar to the case of a step-like force command, a command for increasing the force is continuously issued during a period immediately after changing in which a force deviation (i.e., a difference between a force command value (or a desired value) and a force feedback value (or a detected value)) is large, and as a result, the force command value after collision tends to become excessively large. Thus, the force control for the servo-motor in the conventional die cushion mechanism has a problem that the overshoot of the force tends to become excessively large.